A blue moon never comes. Moon can never be blue. But in English we have a proverb, "once in a blue moon". This means in other words something which is very rare.
Relationships need oil to nurture. If oil is not regularly put, suddenly we find the noise of cracks. It's not smooth anymore, there is friction. The oil we refer to in relationships refer to 2 things - 1. Remembrance & 2. Respect. These are the 2 glues which keep relationships close and well knit.
Let's try and understand the 1st part. The 1st part is "remembrance". This means to stay in touch. If you are not in touch with someone, then one can't really say that you have a relationship worthy of any name. In today's age, we don't necessarily have to meet someone to stay in touch. The mobile messaging world ensures ways in which closeness, and touch can be maintained. If you value a relationship, value a person, then you ensure you keep in touch. In many relationships you have nothing to gain. It does not mean you give it less importance. In those relationships you have the role of a "giver", it's a holy duty.
It's not right to remember people only when you have a need. Friends are there to help, but friends are not there only to help. When you take relationships for granted, when you forget to put the oil needed for smoothness, when you say, "I remembered him once in a blue moon", you are basically sending a signal that the relationship really does not matter. Think once more of the people who matter, have you treated them as if they don't matter ? If yes, then put the oil back, apply it regularly, remember them.